A. Field of the Invention
This invention relates to the construction art, and more particularly to a method of forming underground foundations with liquid tight joints.
B. Description of the Prior Art
There are numerous requirements for in-ground, water impermeable foundations in the construction trade. For example, trenches can be dug in the ground and filled with concrete to halt water seepage, or to allow an excavation to proceed without seriously upsetting the geology of the surrounding land. Water tight construction is also highly desirable for building foundations. When the trenches or foundation sites are dug through solid clay or rock, the sides of the excavation are usually solid enough to enable the achievement of a considerable depth with either a back hoe or a drag-line digging machine. However, such conditions do not ordinarily exist where the ground is saturated with water and is composed of silt, sand and/or gravel.
If the water table is reached while attempting to dig a deep, narrow trench in a saturated sandy soil, the movement of water into the trench loosens the sand and collapses the structure, causing the entire formation to fall. In order to reach the desired depth, the trench may ultimately have to be made so wide on the surface as to render the cost of filling prohibitive.
This problem has been substantially overcome in the past by a technique commonly known as slurry trenching or diaphragm wall construction. This construction technique involves keeping the trench filled during digging with a slurry formed from a mixture of bentonite and water. As digging proceeds and more slurry is added, a tough, flexible seal forms on the inside of the trench and stops the flow of water through the trench walls. This film, together with the pressure of the slurry behind it, holds the walls upright, and water tight. During continued digging, the slurry moves into the freshly exposed dirt and commences its sealing action.
When the trench has been dug to the desired depth and filled with slurry, metal pipes spanning the width of the trench are lowered into one section and act as lateral stops to isolate the section during its subsequent formation. The section is then filled with a water impermeable material such as concrete, clay, or a mixture of clay, sand and gravel while slurry is simultaneously displaced from the trench section, preferably for recovery and reuse. The foundation material is then allowed to harden until the pipes at either end can be removed without the newly formed wall collapsing into the remainder of the trench. After removal of the pipes, another section adjacent to the first section is formed by placing a pipe at the far end of the new section, and filling the new section with foundation material while simultaneously displacing the slurry therein. After the new section has hardened sufficiently, additional sections can be formed in the same manner until the desired wall length is attained. With the use of this method and sophisticated digging equipment, depths of several hundred feet have been achieved.
Bentonite, and specifically sodium bentonite, has a property which makes it particularly useful as the agent for forming the slurry. When dispersed in water, sodium bentonite absorbs water onto the surface of its platelets, giving rise to a multitude of individual platelets of clay, each surrounded by a water jacket. This water jacket gives bentonite the ability to develop viscosity in water, and also to line the trench walls with a tough, thin film which reduces water permeablility.
While the above technique represents a marked advance over previous methods, there is still room for further improvement. Specifically, when a new section of concrete is poured it does not form a perfect bond with the previously poured section, at least partly because the previous section retains a slurry coating. This creates an area of potential leakage for water to seep through the wall along the joint between adjacent sections. In some cases the seepage problem has been severe enough to require the construction of additional wall sections parallel to the primary foundation and adjacent each joint to stop leakage through the joints.
Another method for inhibiting leakage is disclosed in U.S. Pat. No. 3,442,627 to Courte. In one embodiment, described in connection with FIGS. 4 and 5 of the Courte patent, a new foundation section or panel is initially filled with mud and a "key tube" inserted in the mud against an adjacent concrete section. When the new section has been cast with concrete and set, the key tube is removed, leaving behind a recess full of mud. It is next necessary to empty the recess of mud and wash its walls, following an optional step of boring to enlarge the recess and cut through the surface joining the two foundation sections. The recess may then be injected under pressure with a suspension of cement to make it water tight.
An alternate embodiment is described in connection with FIGS. 6-9 of the Courte patent. In this embodiment, a first key tube is held upright in the first foundation section to be cast by attachment to a stop end tube positioned at the far end of the section. After the section has been filled with concrete, the first key tube is disconnected from the stop end tube and both tubes are removed independently from the trench. A guiding tube is then inserted into the recess left by removal of the first key tube. Attached to and guided by the guiding tube is a second key tube which is held in the next foundation section tangentially to the first key tube recess. The second foundation section is then cast with concrete, after which the guiding tube and second key tube are removed to leave adjacent recesses in the first and second foundation sections which open into each other through a restricted opening defined by concrete projections or tongues.
In the next step of this embodiment of Courte, cake is required to be removed by an emulsifier and, if necessary, by sweeping each of the recesses in its turn by flue brushes operating in the recesses in alternation with the emulsifier. The projections or tongues restricting the opening between the two recesses may then be cut off by a boring bit, resulting in an oval cross-section for the recesses. The dual recess is then checked for fluid tightness and cleaned, and a suitable reinforcement layed if required. Finally, the recess is cast with concrete.
While the approach of the Courte patent is theoretically sound, there are practical limitations to its application, and it has been found to be unnecessarily complicated. For example, in each embodiment special attention must be paid to proper positioning of the key tubes. Thus, for the first embodiment the technique of casting sand into the key tube to prevent it from shifting and prevent interior concrete from rising is described, while for the second embodiment various methods of securing the second key tube to the guiding tube are described.
Since each of the key tubes of Courte is set in concrete and must be withdrawn to create a recess, it is important that they be positioned vertically. In certain situations the concrete may freeze around the key tube and make it virtually impossible to withdraw. In other cases, the contractor may choose to "break" the key tube away after the concrete has set in order to withdraw it. In this section, the tearing of the tube out of the concrete could destroy the edge of the concrete section. Furthermore, in both Courte embodiments a mud residue is left in the recess and must be removed to make the joint between sections reliably fluid tight.
The Courte approach is further limited because of the shrinkage characteristics exhibited by the preferred sealing material for foundation walls, which is a grout consisting essentially of water and Portland cement. When this grout is fluid enough to pump readily, it has high shrinkage characteristics. Shrinkage of the grout after the foundation has been completed may open a space between the grout and the recess it has filled, resulting in additional leakage between foundation sections. Accordingly, there is still a need for an improved method of forming an in-ground foundation that is internally waterproof.